Composition

ABSTRACT

A composition provides a high-bulk cellulose fiber-containing sheet, in which the water-retaining ability of the cellulose fibers is sufficiently high, and the water-absorbing rate is large. The composition contains cellulose fibers having phosphoric acid groups or phosphoric acid group-derived substituents. In at least a part of the cellulose fibers, the phosphoric acid groups or the phosphoric acid group-derived substituents are crosslinked. The number of crosslinking points in the cellulose fibers, which is calculated according to the following Equation (1), is 0.20 mmol/g or more, and the water content is 50% by mass or less, with respect to the total mass of the composition: Number of crosslinking points=(amount of strongly acidic groups contained in cellulose fibers−amount of weakly acidic groups contained in cellulose fibers)/2 . . . Equation (1).

TECHNICAL FIELD

The present invention relates to a composition. Specifically, thepresent invention relates to a cellulose fiber-containing compositioncomprising cellulose fibers having phosphoric acid groups.

BACKGROUND ART

Conventionally, cellulose fibers have been broadly utilized in clothes,absorbent articles, paper products, and the like. As cellulose fibers,ultrafine cellulose fibers having a fiber diameter of 1 μm or less havebeen known, as well as cellulose fibers having a fiber diameter of 10 μmor more and 50 μm or less.

For example, when cellulose fibers are used in an absorbent article, thecellulose fibers that are in the form of a non-woven fabric or the likeconstitute various types of members of the absorbent article. In such acase, the non-woven fabric is required to have high water absorbency.Conventionally, in order to enhance the water absorbency of a non-wovenfabric comprising cellulose fibers, a highly water-absorbent resin suchas SAP has been deposited on the cellulose fibers. In addition, atechnique of performing crosslinking modification on cellulose fibers toenhance the water absorbency of a non-woven fabric has also beenstudied.

Patent Document 1 discloses a fabric cloth containing crosslink-modifiedcellulose fibers. Herein, formaldehyde, a nitrogen-containing cycliccompound and the like are used as crosslinkers in the crosslinkingmodification. Moreover, Patent Document 2 discloses a method forproducing a water-absorbent cellulose material having an excellent saltwater-absorbing rate, which comprises immersing a water-swellable,crosslinked cellulose derivative having a carboxyl group in a stronglyacidic aqueous solution, then adding alkali to the cellulose derivativein an organic solvent having compatibility with water so that theacid-type carboxyl group is converted to a salt type, and then dryingthe resultant. Also in this method, a technique of crosslinkingcellulose fibers to enhance the water absorption of the cellulosematerial has been studied.

Patent Document 3 discloses a water-absorbent resin comprising celluloseand a polymer obtained by polymerizing a monomer aqueous solution havingan acid group-containing unsaturated monomer as an essential component,wherein the resin has a crosslinked surface. Patent Document 3 describesphosphorylated crosslinked cellulose as cellulose, and also describes,as a crosslinker, an N-methylol compound such as dimethylol ethyleneurea or dimethylol dihydroxy ethylene urea.

PRIOR ART DOCUMENTS Patent Documents Patent Document 1: JP-A-2000-129575Patent Document 2: JP-A-H08-243388 (1996) Patent Document 3:JP-A-2011-213759 SUMMARY OF INVENTION Object to be Solved by theInvention

In general, when cellulose fibers are crosslinked, a cellulosefiber-containing sheet tends to have high bulk. However, in suchhigh-bulk cellulose fiber-containing sheet, the water-retaining abilityof cellulose fibers tends to be decreased. Thus, it has been desired todevelop a cellulose fiber-containing sheet having sufficiently highwater-retaining ability, although the sheet has high bulk. In addition,studies have also been conducted to enhance the water-absorbing rate ofsuch a cellulose fiber-containing sheet.

Hence, in order to solve the problems of the prior art techniques, thepresent inventors have conducted studies directed towards providing acellulose fiber-containing composition having sufficiently highwater-retaining ability and being capable of exhibiting an excellentwater-absorbing rate, even in the case of forming a high-bulk sheet fromthe cellulose fiber-containing composition.

Means for Solving the Object

As a result of intensive studies directed towards achieving theaforementioned object, the present inventors have found that the amountof the crosslinked structures of phosphorylated cellulose fibers in acomposition comprising the phosphorylated cellulose fibers havingcrosslinked structures is set to be a predetermined amount or larger, sothat sufficiently high water-retaining ability and an excellentwater-absorbing rate can be exhibited, even in a case where a high-bulksheet is formed from the aforementioned composition.

Specifically, the present invention has the following configurations.

[1] A composition comprising cellulose fibers having phosphoric acidgroups or phosphoric acid group-derived substituents, wherein

in at least a part of the cellulose fibers, the phosphoric acid groupsor the phosphoric acid group-derived substituents are crosslinked,

the number of crosslinking points in the cellulose fibers, which iscalculated according to the following Equation (1), is 0.20 mmol/g ormore, and

the water content is 50% by mass or less, with respect to the total massof the composition:

Number of crosslinking points=(amount of strongly acidic groupscontained in cellulose fibers−amount of weakly acidic groups containedin cellulose fibers)/2   Equation (1).

[2] The composition according to [1], which is a non-woven fabric.[3] The composition according to [1] or [2], wherein when thecomposition is processed into a rectangular sample having a width of 5mm and a length of 50 mm, then, an edge region ranging from the end ofthe rectangular sample in the longitudinal direction to 5 mm from theend is immersed in ion exchange water (electrical conductivity: 2 ρS/cmor less), and then, the time required for the ion exchange water toreach from the end of the longitudinal direction to a distance of 45 mmin the longitudinal direction is measured, a water-absorbing rate(mm/sec), which is calculated according to the following Equation (2),is 2.5 mm/sec or more and 100 mm/sec or less:

Water-absorbing rate (mm/sec)=40 (mm)/t (sec)  Equation (2)

wherein t represents the time (see) required for the ion exchange waterto reach from the end of the rectangular sample in the longitudinaldirection to a distance of 45 mm in the longitudinal direction.

[4] The composition according to any one of [1] to [3], wherein theamount of the strongly acidic groups contained in the cellulose fibersis 1.60 mmol/g or more.[5] The composition according to any one of [1] to [4], wherein thewater retention capacity (%) of the cellulose fibers, which iscalculated according to the following equation, is 150% or more:

Water retention capacity (%)=(weight of cellulose fibers aftercentrifugation treatment−absolute dry weight of cellulosefibers)/absolute dry weight of cellulose fibers×100,

wherein, in the above equation, the water retention capacity is measuredin accordance with SCAN-C 62:00, and conditions for the centrifugationtreatment are determined to be 20° C. and weight acceleration upon thecentrifugation of 3950 g, and 15 minutes.

Advantageous Effects of Invention

According to the present invention, a cellulose fiber-containingcomposition having sufficiently high water-retaining ability and beingcapable of exhibiting an excellent water-absorbing rate even in a casewhere a high-bulk sheet is formed from the composition can be obtained.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing the relationship between the amount of NaOHadded dropwise to a fiber raw material and the pH.

EMBODIMENTS OF CARRYING OUT THE INVENTION

hereinafter, the present invention will be described in detail. Thebelow-mentioned constituent features will be explained based onrepresentative embodiments or specific examples in some cases. However,the present invention is not limited to such embodiments.

(Composition)

The present invention relates to a composition comprising cellulosefibers having phosphoric acid groups or phosphoric acid group-derivedsubstituents (hereinafter simply referred to as “phosphoric acid groups”at times). Herein, phosphoric acid groups or phosphoric acidgroup-derived substituents are crosslinked in at least a part of thecellulose fibers. The number of crosslinking points in the cellulosefibers, which is calculated according to the following Equation (1), is0.20 mmol/g or more.

Number of crosslinking points=(amount of strongly acidic groupscontained in cellulose fibers−amount of weakly acidic groups containedin cellulose fibers)/2   Equation (1)

In addition, the water content is 50% by mass or less, with respect tothe total mass of the composition of the present invention. Besides, inthe present description, the composition comprising cellulose fibers canbe referred to as a “cellulose fiber-containing composition.”

Since the cellulose fiber-containing composition of the presentinvention has the above-described configuration, it can form a high-bulkcellulose fiber-containing sheet. However, in such a case as well, thewater-retaining ability of the cellulose fibers can be kept atsufficiently high. Moreover, when the cellulose fiber-containingcomposition of the present invention is processed into a sheet, thesheet can exhibit an excellent water-absorbing rate. In the presentinvention, by appropriately regulating the crosslinked structures ofphosphorylated cellulose fibers or the amount of the crosslinkedstructures, a cellulose fiber-containing composition capable ofexhibiting novel physical properties has been successfully obtained.

The water content is 50% by mass or less, with respect to the total massof the cellulose fiber-containing composition of the present invention.This means that the cellulose fiber-containing composition of thepresent invention is preferably, not in a slurry state but in a solidstate. For example, the cellulose fiber-containing composition of thepresent invention is preferably in the state of gel, a sheet or aparticulate, and more preferably in the state of a sheet. Among others,the cellulose fiber-containing composition of the present invention ispreferably a non-woven fabric. Besides, in the present description, whenthe cellulose fiber-containing composition is in a sheet state, thecellulose fiber-containing composition can also be referred to as a“cellulose fiber-containing sheet.” The cellulose fiber-containing sheetis one embodiment of the cellulose fiber-containing composition.

The water content with respect to the total mass of the cellulosefiber-containing composition of the present invention may be 50% by massor less, preferably 40% by mass or less, more preferably 30% by mass orless, further preferably 20% by mass or less, and particularlypreferably 15% by mass or less. In addition, in the present invention,the water content in the cellulose fiber-containing composition may alsobe 0% by mass.

Herein, the water content can be calculated as follows. That is, theweight of a cellulose fiber-containing composition, which has beensubjected to humidity conditioning up to an equilibrium state underconditions of 23° C. and a relative humidity of 50%, is measured, thecellulose fiber-containing composition is then dried at 105° C.overnight, and the weight of the resulting cellulose fiber-containingcomposition is then measured. Then, the water content can be calculatedaccording to the following equation:

Water content (%)=(weight of cellulose fiber-containing compositionbefore drying at 105° C.−weight of cellulose fiber-containingcomposition after drying at 105° C.)/weight of cellulosefiber-containing composition before drying at 105° C.×100.

When the cellulose fiber-containing composition of the present inventionis in a sheet state, the density of the cellulose fiber-containing sheetis preferably 1.2 g/cm³ or less, more preferably 1.0 g/cm³ or less, andfurther preferably 0.8 g/cm³ or less. On the other hand, the density ofthe cellulose fiber-containing sheet is preferably 0.05 g/cm³ or more.Even in a case where the cellulose fiber-containing composition of thepresent invention is a non-woven fabric, the density of the non-wovenfabric is preferably within the above-described range. When thecellulose fiber-containing composition of the present invention is asheet, the density of the cellulose fiber-containing sheet is preferablywithin the above-described range, and a high-bulk sheet can be obtainedby adjusting the density within the above-described range.

When the cellulose fiber-containing composition of the present inventionis a sheet, the basis weight of the cellulose fiber-containing sheet ispreferably 30 g/m² or more, more preferably 50 g/m² or more, and furtherpreferably 100 g/m² or more. On the other hand, the basis weight of thecellulose fiber-containing sheet is preferably 1000 g/m² or less. Bysetting the basis weight of the cellulose fiber-containing sheet withinthe above-described range, water absorbency can be more effectivelyexhibited.

When the cellulose fiber-containing composition of the present inventionis a sheet, the thickness of the cellulose fiber-containing sheet ispreferably 5 μm or more, more preferably 10 μm or more, and furtherpreferably 15 μm or more. On the other hand, the thickness of thecellulose fiber-containing sheet is preferably 50 mm or less, morepreferably 40 mm or less, and further preferably 30 mm or less.

When the cellulose fiber-containing composition of the present inventionis a sheet, the water-absorbing rate (mm/sec) calculated according tothe following Equation (2) is preferably 2.5 mm/sec or more and 100mm/sec or less. The water-absorbing rate (mm/sec) is more preferably 3.0mm/sec or more, and further preferably 3.5 mm/sec or more.

Herein, the water-absorbing rate (mm/sec) calculated according to thefollowing Equation (2) is a water-absorbing rate measured by thefollowing procedures. First, a sheet-like cellulose fiber-containingcomposition is processed into a rectangular sample having a width of 5mm and a length of 50 mm, and an edge region ranging from the end ofthis rectangular sample in the longitudinal direction to 5 mm from theend is then immersed in ion exchange water (electrical conductivity: 2μS/cm or less). Thereafter, the time required for the ion exchange waterto reach from the end of the longitudinal direction to a distance of 45mm in the longitudinal direction is measured. After that, awater-absorbing rate (mm/sec) is calculated from the obtained timeaccording to the following Equation (2):

Water-absorbing rate (mm/sec)=40 (mm)/t (sec)  Equation (2).

In the above Equation (2), t represents the time (sec) required for theion exchange water to reach from the end of the rectangular sample inthe longitudinal direction to a distance of 45 mm in the longitudinaldirection.

(Cellulose Fibers)

The cellulose fiber-containing composition of the present inventioncomprises, as main components, cellulose fibers having phosphoric acidgroups. Herein, the state in which cellulose fibers having phosphoricacid groups are comprised as main components in the cellulosefiber-containing composition means that the content of the cellulosefibers having phosphoric acid groups is 50% by mass or more, withrespect to the total mass of the cellulose fiber-containing composition.The content of the cellulose fibers having phosphoric acid groups ispreferably 60% by mass or more, more preferably 70% by mass or more, andfurther preferably 80% by mass or more, with respect to the total massof the cellulose fiber-containing composition.

The cellulose raw material for obtaining cellulose fibers is notparticularly limited, but pulp is preferably used from the viewpoint ofavailability and inexpensiveness. Examples of the pulp include woodpulp, non-wood pulp, and deinked pulp. Examples of the wood pulp includechemical pulps such as leaf bleached kraft pulp (LBKP), needle bleachedkraft pulp (NBKP), sulfite pulp (SP), dissolving pulp (DP), soda pulp(AP), unbleached kraft pulp (UKP), and oxygen bleached kraft pulp (OKP).Further, included are, but not particularly limited to, semichemicalpulps such as semi-chemical pulp (SCP) and chemi-ground wood pulp (CGP);and mechanical pulps such as ground pulp (GP) and thermomechanical pulp(TMP, BCTMP). Examples of the non-wood pulp include, but notparticularly limited to, cotton pulps such as cotton linter and cottonlint; non-wood type pulps such as hemp, wheat straw, and bagasse; andcellulose isolated from ascidian, seaweed, etc., chitin, and chitosan.As a deinked pulp, there is deinked pulp using waste paper as a rawmaterial, but it is not particularly limited thereto. The pulp of thepresent embodiment may be used singly, or in combination of two or moretypes. Among the above-listed pulp types, wood pulp and deinked pulpincluding cellulose are preferable from the viewpoint of easyavailability.

In the present invention, the fiber width of a cellulose fiber having aphosphoric acid group is not particularly limited. For example, thefiber width of a cellulose fiber having a phosphoric acid group may begreater than 1000 nm, or may also be 1000 nm or less. Moreover,cellulose fibers having a fiber width of greater than 1000 nm may bepresent together with cellulose fibers having a fiber width of 1000 nmor less. When the fiber width of a cellulose fiber is 1000 nm or less,such a cellulose fiber may also be referred to as an “ultrafinecellulose fiber.”

Moreover, the cellulose fiber-containing composition of the presentinvention may also comprise cellulose fibers having no phosphoric acidgroups, as well as cellulose fibers having phosphoric acid groups. Inthis case, the content of the cellulose fibers having no phosphoric acidgroups is preferably 20% by mass or less, and more preferably 10% bymass or less, with respect to the total mass of the fiber raw material.

Herein, the fiber width of a cellulose fiber can be measured by electronmicroscopic observation according to the following method. First, anaqueous suspension of cellulose fibers having a concentration of 0.05%by mass or more and 0.1% by mass or less is prepared, and the suspensionis casted onto a hydrophilized carbon film-coated grid as a sample forTEM observation. At this time, SEM images of the surface of thesuspension casted onto glass may be observed. The sample is observedusing electron microscope images taken at a magnification of 1000×,5000×, 10000×, or 50000×, depending on the widths of the constituentfibers. However, the sample, the observation conditions, and themagnification are adjusted so as to satisfy the following conditions:

(1) A single straight line X is drawn in any given portion in anobservation image, and 20 or more fibers intersect with the straightline X.

(2) A straight line Y, which intersects perpendicularly with theaforementioned straight line in the same image as described above, isdrawn, and 20 or more fibers intersect with the straight line Y.

The widths of the fibers intersecting the straight line X and thestraight line Y in the observation image meeting the above-describedconditions are visually read. Three or more sets of images of surfaceportions, which are at least not overlapped, are thus observed, and thewidths of the fibers intersecting the straight line X and the straightline Y are read in the each image. At least 120 fiber widths (20fibers×2×3=120) are thus read.

The average fiber length of cellulose fibers is not particularlylimited, but it is preferably 0.1 mm or more, and more preferably 0.6 mmor more. On the other hand, it is preferably 5 mm or less, and morepreferably 2 mm or less. By setting the average fiber length ofcellulose fibers within the above-described range, when the cellulosefiber-containing composition is processed into a sheet, the strength ofthe sheet can be enhanced. Herein, the average fiber length of cellulosefibers can be obtained, for example, by using Kajaani Fiber SizeAnalyzer (FS-200) manufactured by Kajaani Automation to measure thelength weighted average fiber length. Otherwise, the average fiberlength of cellulose fibers may also be measured by using a scanningelectron microscope (SEM), a transmission electron microscope (TEM),etc., depending on the length of the fiber.

When the cellulose fibers are ultrafine cellulose fibers, the ultrafinecellulose fibers preferably have a type I crystal structure. In thisregard, the fact that ultrafine cellulose fibers have a type I crystalstructure may be identified by a diffraction profile obtained from awide angle X-ray diffraction photograph using CuKα (λ=1.5418 Å)monochromatized with graphite. Specifically, it may be identified basedon the fact that there are typical peaks at two positions near 2θ=14° ormore and 17° or less, and near 2θ=22° or more and 23° or less.

The percentage of the type I crystal structure occupied in the ultrafinecellulose fibers is preferably 30% or more, more preferably 50% or more,and further preferably 70% or more. In this case, more excellentperformance can be expected, in terms of heat resistance and theexpression of low linear thermal expansion. The crystallinity can beobtained by measuring an X-ray diffraction profile and obtaining itaccording to a common method (Seagal et al., Textile Research Journal,Vol. 29, p. 786, 1959).

In the present description, cellulose fibers have phosphoric acid groups(phosphoric acid groups or phosphoric acid group-derived substituents).In the present invention, such cellulose fibers may also be referred toas “phosphorylated cellulose fibers.”

The phosphoric acid group comprised in the phosphorylated cellulosefibers is a divalent functional group corresponding to a phosphoricacid, from which a hydroxyl group is removed. Specifically, it is agroup represented by —PO₃H₂. The phosphoric acid group-derivedsubstituents include substituents, such as condensation-polymerizedphosphoric acid groups, salts of phosphoric acid groups, and phosphoricacid ester groups, and they may preferably be ionic substituents.

In the present invention, the phosphoric acid group or the phosphoricacid group-derived substituent may be a substituent represented by thefollowing Formula (1):

In the Formula (1), a, b, and n each represent a natural number(provided that a=b×m); at least one of α1, α2, . . . , αn and α′ is O⁻,and the rest are either R or OR. All of αn and α′ may also be O⁻. When nis 2 or greater and α′ is R or OR, at least one of αn is O⁻ and the restare R or OR. When n is 2 or greater and α′ is O⁻, all of an may be R orOR, or at least one of an may be O⁻ and the rest may be R or OR. R eachrepresents a hydrogen atom, a saturated straight chain hydrocarbongroup, a saturated branched chain hydrocarbon group, a saturated cyclichydrocarbon group, an unsaturated straight chain hydrocarbon group, anunsaturated branched chain hydrocarbon group, an unsaturated cyclichydrocarbon group, an aromatic group, or a derivative group thereof.

Examples of the saturated straight chain hydrocarbon group include amethyl group, an ethyl group, an n-propyl group, and an n-butyl group,but are not particularly limited thereto. Examples of the saturatedbranched chain hydrocarbon group include an i-propyl group and a t-butylgroup, but are not particularly limited thereto. Examples of thesaturated cyclic hydrocarbon group include a cyclopentyl group and acyclohexyl group, but are not particularly limited thereto. Examples ofthe unsaturated straight chain hydrocarbon group include a vinyl groupand an allyl group, but are not particularly limited thereto. Examplesof the unsaturated branched chain hydrocarbon group include ani-propenyl group and a 3-butenyl group, but are not particularly limitedthereto. Examples of the unsaturated cyclic hydrocarbon group include acyclopentenyl group and a cyclohexenyl group, but are not particularlylimited thereto. Examples of the aromatic group include a phenyl groupand a naphthyl group, but are not particularly limited thereto.

Moreover, examples of the derivative of the above-described R includefunctional groups such as a carboxyl group, a hydroxyl group or an aminogroup, in which at least one type selected from functional groups isadded to or substituted with the main chain or side chain of theabove-described various types of hydrocarbon groups, but are notparticularly limited thereto. Furthermore, the number of carbon atomsconstituting the main chain of the above-described R is not particularlylimited, but it is preferably 20 or less, and more preferably 10 orless. If the number of carbon atoms constituting the main chain of the Rexceeds 20, the molecules of phosphorus oxoacid groups containing Rbecome too large, the groups can hardly permeate into a fiber rawmaterial, so that the yield of ultrafine cellulose fibers is likely tobe decreased.

βb+ is a mono- or more-valent cation consisting of an organic orinorganic matter. Examples of the mono- or more-valent cation consistingof an organic matter include an aliphatic ammonium and an aromaticammonium, and examples of the mono- or more-valent cation consisting ofan inorganic matter include alkali metal ions such as sodium, potassiumor lithium ions, divalent metal cations such as calcium or magnesiumions, and hydrogen ions, but are not particularly limited thereto. Thesecan be applied alone as a single type or in combination of two or moretypes. As such mono- or more-valent cations consisting of an organic orinorganic matter, sodium or potassium ions, which hardly cause theyellowing of a fiber raw material containing β upon heating and areindustrially easily applicable, are preferable, but are not particularlylimited thereto.

The content of the phosphoric acid groups comprised in the cellulosefibers is preferably 0.10 mmol/g or more, more preferably 0.20 mmol/g ormore, even more preferably 0.50 mmol/g or more, further preferably 1.00mmol/g or more, still further preferably 1.20 mmol/g or more,particularly preferably 1.30 mmol/g or more, and most preferably 1.60mmol/g or more, per gram (mass) of the cellulose fibers. On the otherhand, the content of the phosphoric acid groups is preferably 3.65mmol/g or less, more preferably 3.5 mmol/g or less, and furtherpreferably 3.0 mmol/g or less. Besides, in the present description, thecontent of the phosphoric acid groups comprised in the cellulose fibersis equal to the amount of strongly acidic groups of phosphoric acidgroups in the cellulose fibers, as described later.

The content of the phosphoric acid groups in the cellulose fibers can bemeasured by a neutralization titration method. Upon the measurement bysuch a neutralization titration method, phosphoric acid groups arecompletely converted to acid type groups, and fibrillation is thenperformed by a mechanical treatment step (fibrillation step).Thereafter, while a sodium hydroxide aqueous solution is added to theobtained ultrafine cellulose fiber-containing slurry, changes in the pHare obtained, so that the amount of phosphoric acid groups introducedcan be measured.

Conversion of the phosphoric acid groups to acid type groups is carriedout by diluting the obtained phosphorylated cellulose fibers with ionexchange water, so that the concentration of cellulose fibers becomes 2%by mass, and then gradually adding a sufficient amount of 1 Nhydrochloric acid aqueous solution to the resulting phosphorylatedcellulose fibers, while stirring. In such conversion of the phosphoricacid groups to acid type groups, it is preferable to repeat theoperation of dehydrating the above-described cellulose fiber-containingslurry to obtain a dehydrated sheet, then diluting the dehydrated sheetwith ion exchange water again, and then adding a 1 N hydrochloric acidaqueous solution to the resultant, so that the phosphoric acid groupscontained in the cellulose fibers can be completely converted to acidtype groups. Then, after completion of the step of converting thephosphoric acid groups to acid type groups, it is preferable to repeatthe operation of stirring the obtained cellulose fiber-containing slurryto uniformly disperse it, followed by filtration and dehydration toobtain a dehydrated sheet, so that redundant hydrochloric acid can befully washed away.

In the mechanical treatment step (fibrillation step), ion exchange wateris poured onto the obtained dehydrated sheet to obtain a cellulosefiber-containing slurry, in which the concentration of cellulose fibersis 0.3% by mass, and this slurry is then treated using a defibrationtreatment device (manufactured by M Technique Co., Ltd., CLEARMIX-2.2S)under conditions of 21500 rotations/min for 30 minutes. Thus, anultrafine cellulose fiber-containing slurry is obtained.

In the titration using alkali, changes in the pH values indicated by thedispersion are measured while adding a 0.1 N sodium hydroxide aqueoussolution to the ultrafine cellulose fiber-containing slurry. In thisneutralization titration, in a curve obtained by plotting pH valuesmeasured with respect to the amount of alkali (sodium hydroxide aqueoussolution) added, two points, in which the increment (the derivative ofpH to the amount of alkali added dropwise) becomes maximum, are obtained(i.e., a point in which the increment becomes maximum, and a point inwhich the increment becomes second maximum). Among these, the amount ofalkali required until the maximum point of the increment obtained firstafter addition of alkali (hereinafter referred to as a “first endpoint”) is equal to the amount of strongly acidic groups in thedispersion used in the titration, and the amount of alkali requireduntil the maximum point of the increment obtained second after additionof alkali (hereinafter referred to as a “second end point”) is equal tothe amount of weakly acidic groups in the dispersion used in thetitration. The alkali amount (mmol) required until the first end pointis divided by the solid content (g) in the ultrafine cellulosefiber-containing slurry to be titrated, to obtain a first dissociatedalkali amount (mmol/g). This amount is defined to be the content of thephosphoric acid groups in the cellulose fibers.

FIG. 1 shows a curve obtained by plotting the pH values measured withrespect to the amount of alkali (sodium hydroxide aqueous solution) inneutralization titration. The region up to the first end point isreferred to as a first region, and the region up to the second end pointis referred to as a second region. Besides, after the second region,there is a third region. In short, three regions appear. In FIG. 1, theamount of the alkali required for the first region is equal to theamount of strongly acidic groups in the slurry used in the titration,and the amount of the alkali required for the second region is equal tothe amount of weakly acidic groups in the slurry used in the titration.

It is considered that a crosslinked structure is formed by dehydrationcondensation of phosphoric acid groups introduced into cellulose fibers.Specifically, the crosslinked structure is a structure in which glucoseunits of cellulose individually bind to each one of two P atoms ofpyrophosphoric acid via an O atom. Accordingly, when such a crosslinkedstructure is formed, weakly acidic groups are apparently lost, and thus,the amount of alkali required up to the second end point is reduced incomparison to the amount of alkali required up to the first end point.Herein, if phosphoric acid groups introduced into cellulose fibers arenot condensed at all, the amount of strongly acidic groups introducedinto cellulose fibers is equal to the amount of weakly acidic groupsintroduced into cellulose fibers. As such, the value obtained bydividing the amount of weakly acidic groups lost as a result of theformation of the crosslinked structure by 2 indicates the amount ofcrosslinked structures (the number of crosslinking points). That is, theamount of the crosslinked structures (the number of crosslinking points)is equal to the value obtained by dividing a difference between theamount of alkali required up to the first end point (first dissociatedalkali amount) and the amount of alkali required up to the second endpoint (second dissociated alkali amount) by 2. The amount of thecrosslinked structures (the number of crosslinking points) isrepresented by the following Equation (1):

Amount of crosslinked structures (number of crosslinking points)=(amountof strongly acidic groups contained in cellulose fibers−amount of weaklyacidic groups contained in cellulose fibers)/2  Equation (1)

In the present invention, the amount of crosslinked structures (thenumber of crosslinking points) in cellulose fibers, which is calculatedby the above Equation (1), may be 0.20 mmol/g or more, preferably 0.22mmol/g or more, and more preferably 0.25 mmol/g or more. Besides, theupper limit value of the amount of the crosslinked structures (thenumber of crosslinking points) is a value obtained by dividing theamount of strongly acidic groups contained in cellulose fibers by 2.Thus, the upper limit value may be, for example, 1.82 mmol/g or less.

The water retention capacity of cellulose fibers is preferably 150% ormore, more preferably 170% or more, and further preferably 200% or more.The upper limit value of the water retention capacity of cellulosefibers is not particularly limited, but it can be set, for example, at1000%. Herein, the water retention capacity of cellulose fibers is avalue measured in accordance with SCAN-C 62:00, and this value iscalculated according to the equation below. Upon the measurement of thewater retention capacity of cellulose fibers, the cellulose fibers aresubjected to a centrifugation treatment for 15 minutes under conditionsof 20° C. and 4400 rpm (weight acceleration upon centrifugation: 3950g). The amount of cellulose fibers subjected to a centrifugationtreatment is set at 0.5 g (absolute dry weight) for a single measurement(added basis weight: 1700±100 g/m²). As a centrifugal separator usedherein, for example, H-3R manufactured by KOKUSAN Co. Ltd. can be used.It is to be noted that the larger the numerical value of water retentioncapacity, the higher the affinity of cellulose fibers with water thatcan be obtained.

Water retention capacity (%)=(weight of cellulose fibers aftercentrifugation treatment−absolute dry weight of cellulosefibers)/absolute dry weight of cellulose fibers×100

Cellulose fibers may have counterions. Such counterions may be eitherinorganic ions or organic ions. Examples of the inorganic ions include:monovalent metal ions including alkali metal ions as representativeexamples; divalent metal ions including alkaline earth metal ions asrepresentative examples; and other non-metal cations including basemetal ions such as ammonium ions, aluminum ions, tin ions or lead ions,and transition metal ions such as silver ions, copper ions, or ironions. Examples of the organic ions include organic ammonium ions andorganic phosphonium ions. When water retention capacity intends to beenhanced, monovalent cations are preferably used as counterions. Fromthe viewpoint of versatility, ammonium ions and alkali metal ions aremore preferably used as counterions, and sodium ions and ammonium ionsare further preferably used as counterions. On the other hand, whenfunctions such as deodorant and antibacterial functions intend to beimparted, functional cations such as copper ions, silver ions, ororganic ammonium ions are preferably used as counterions.

(Optional Component)

The cellulose fiber-containing composition of the present invention maycomprise optional components other than cellulose fibers. Examples ofsuch optional components may include antifoaming agents, lubricants,surfactants, ultraviolet absorbing agents, dyes, pigments, fillers,stabilizers, organic solvents miscible with water, such as alcohol,antiseptics, organic fine particles, inorganic fine particles, andresins (pellet-type and fibrous resins).

(Method for Producing Composition)

The method for producing a composition (method for producing a cellulosefiber-containing composition) comprises a step of introducing phosphoricacid groups or phosphoric acid group-derived substituents into cellulosefibers, and then crosslinking the phosphoric acid groups or phosphoricacid group-derived substituents, so that the number of crosslinkingpoints in the cellulose fibers, which is calculated according to thefollowing Equation (1), can be 0.20 mmol/g or more. The water content ofthe thus obtained composition is 50% by mass or less.

Number of crosslinking points=(amount of strongly acidic groupscontained in cellulose fibers−amount of weakly acidic groups containedin cellulose fibers)/2   Equation (1)

<Phosphoric Acid Group Introduction Step>

The step of introducing phosphoric acid groups or phosphoric acidgroup-derived substituents into cellulose fibers may be referred to as a“phosphoric acid group introduction step.” Besides, a step ofcrosslinking at least a part of phosphoric acid groups or phosphoricacid group-derived substituents is included in the phosphoric acid groupintroduction step. That is to say, the phosphoric acid groupintroduction step includes a step of phosphorylating cellulose fibersand a step of crosslinking at least a part of phosphoric acid groups orphosphoric acid group-derived substituents.

The phosphoric acid group introduction step may be performed by allowingat least one selected from a compound having phosphoric acid groups orphosphoric acid group-derived substituents and salts thereof(hereinafter, referred to as a “phosphorylating agent” or “Compound A”)to react with cellulose fibers. Such a phosphorylating agent may bemixed into the cellulose fibers in a dry or wet state, in the form of apowder or an aqueous solution. In another example, a powder or anaqueous solution of the phosphorylating agent may be added into a slurryof the cellulose fibers. That is to say, the phosphoric acid groupintroduction step includes, at least, a step of mixing cellulose fiberswith a phosphorylating agent.

The phosphoric acid group introduction step may be performed by allowinga phosphorylating agent to react with cellulose fibers. This reactionmay be performed in the presence of at least one selected from urea andderivatives thereof (hereinafter, referred to as “Compound B”).

One example of the method of allowing Compound A to act on the cellulosefibers in the presence of Compound B includes a method of mixing thecellulose fibers in a dry or wet state with a powder or an aqueoussolution of Compound A and Compound B. Another example thereof includesa method of adding a powder or an aqueous solution of Compound A andCompound B to a cellulose fiber-containing slurry. Among them, a methodof adding an aqueous solution of Compound A and Compound B to thecellulose fibers in a dry state, or a method of adding a powder or anaqueous solution of Compound A and Compound B to the cellulose fibers ina wet state is preferable because of the high homogeneity of thereaction. Compound A and Compound B may be added at the same time or maybe added separately. Alternatively, Compound A and Compound B to besubjected to the reaction may be first added as an aqueous solution,which may be then compressed to squeeze out redundant chemicals. Theform of the cellulose fibers is preferably a cotton-like or thin sheetform, but the form is not particularly limited thereto.

The phosphorylating agent (Compound A) is at least one selected fromcompounds having phosphoric acid groups and the salts thereof. Examplesof the compound having phosphoric acid groups include, but are notparticularly limited to, phosphoric acid, lithium salts of phosphoricacid, sodium salts of phosphoric acid, potassium salts of phosphoricacid, and ammonium salts of phosphoric acid. Examples of the lithiumsalts of phosphoric acid include lithium dihydrogen phosphate, dilithiumhydrogen phosphate, trilithium phosphate, lithium pyrophosphate, andlithium polyphosphate. Examples of the sodium salts of phosphoric acidinclude sodium dihydrogen phosphate, disodium hydrogen phosphate,trisodium phosphate, sodium pyrophosphate, and sodium polyphosphate.Examples of the potassium salts of phosphoric acid include potassiumdihydrogen phosphate, dipotassium hydrogen phosphate, tripotassiumphosphate, potassium pyrophosphate, and potassium polyphosphate.Examples of the ammonium salts of phosphoric acid include ammoniumdihydrogen phosphate, diammonium hydrogen phosphate, triammoniumphosphate, ammonium pyrophosphate, and ammonium polyphosphate. Amongthese, phosphoric acid, sodium salts of phosphoric acid, potassium saltsof phosphoric acid, and ammonium salts of phosphoric acid are preferablyused.

Since the uniformity of the reaction is improved and the efficiency inintroduction of a phosphoric acid group is further enhanced, thephosphorylating agent (Compound A) is preferably used as an aqueoussolution. Although there is no particular restriction on the pH of anaqueous solution of the phosphorylating agent (Compound A), the pH ispreferably pH 7 or less because the efficiency in introduction ofphosphoric acid groups becomes high, and more preferably pH 3 or moreand pH 7 or less from the viewpoint of suppression of hydrolysis ofcellulose fibers. The pH of an aqueous solution of the Compound A may beadjusted, for example, by using, among compounds having phosphoric acidgroups, a combination of an acidic one and an alkaline one, and changingthe amount ratio thereof. The pH of an aqueous solution of thephosphorylating agent (Compound A) may also be adjusted by adding aninorganic alkali or an organic alkali to an acidic compound amongcompounds having phosphoric acid groups.

The amount of the phosphorylating agent (Compound A) added to cellulosefibers is not particularly limited. However, when the additive amount ofthe phosphorylating agent (Compound A) is converted to the amount ofphosphorus atoms, the amount of the phosphorus atoms added to cellulosefibers (absolute dry mass) is preferably 0.5% by mass or more and 100%by mass or less, more preferably 1% by mass or more and 50% by mass orless, and most preferably 2% by mass or more and 30% by mass or less. Ifthe amount of the phosphorus atoms added to cellulose fibers is withinthe above-described range, the yield of phosphorylated cellulose fiberscan be further improved. By setting the amount of the phosphorus atomsadded to cellulose fibers at 100% by mass or less, a balance can be keptbetween the effect of improving the yield and costs. On the other hand,by setting the amount of the phosphorus atoms added to cellulose fibersat the above-described lower limit value or more, the yield can beenhanced.

Examples of the Compound B used in the present embodiment include urea,biuret, 1-phenyl urea, l-benzyl urea, 1-methyl urea, and i-ethyl urea.

The Compound B is preferably used as an aqueous solution, as with theCompound A. Further, an aqueous solution in which both the Compound Aand Compound B are dissolved is preferably used, because the uniformityof a reaction may be enhanced. The amount of the Compound B added tocellulose fibers (absolute dry mass) is preferably 1% by mass or moreand 500% by mass or less, more preferably 10% by mass or more and 400%by mass or less, further preferably 100% by mass or more and 350% bymass or less, and particularly preferably 150% by mass or more and 300%by mass or less.

The reaction system may contain an amide or an amine, in addition to theCompound A and the Compound B. Examples of the amide include formamide,dimethylformamide, acetamide, and dimethylacetamide. Examples of theamine include methylamine, ethylamine, trimethylamine, triethylamine,monoethanolamine, diethanolamine, triethanolamine, pyridine,ethylenediamine, and hexamethylenediamine. Among them, particularly,triethylamine is known to work as a favorable reaction catalyst.

The phosphoric acid group introduction step preferably has a heatingstep (hereinafter also referred to as a “heat treatment step”). Byestablishing such a heat treatment step, phosphoric acid groups can beefficiently introduced into cellulose fibers, and further, at least apart of phosphoric acid groups or phosphoric acid group-derivedsubstituents can be crosslinked. That is to say, the method forproducing a composition of the present invention preferably comprises astep of mixing cellulose fibers with a phosphorylating agent, and a stepof heating a mixture of the cellulose fibers and the phosphorylatingagent.

With regard to the heat treatment temperature applied in the heattreatment step, it is preferable to select a temperature that allows anefficient introduction of phosphoric acid groups while suppressing thethermal decomposition or hydrolysis reaction of cellulose fibers. Inaddition, with regard to the heat treatment temperature, it ispreferable to select a temperature at which phosphoric acid groups orphosphoric acid group-derived substituents are crosslinked so that thenumber of crosslinking points in cellulose fibers calculated accordingto the aforementioned Equation (1) can be 0.20 mmol/g or more.Specifically, the temperature is preferably 50° C. or higher and 300° C.or lower, more preferably 100° C. or higher and 250° C. or lower, andfurther preferably 130° C. or higher and 200° C. or lower. Moreover, avacuum dryer, an infrared heating device, or a microwave heating devicemay be used for heating.

Upon the heat treatment, if the time for leaving the cellulose fibers tostand still gets longer while the cellulose fiber-containing slurry towhich the Compound A is added contains water, as drying advances, watermolecules and the Compound A dissolved therein move to the surface ofthe cellulose fibers. As such, there is a possibility of the occurrenceof unevenness in the concentration of the Compound A in the cellulosefibers, and the introduction of phosphoric acid groups into thecellulose fiber surface may not progress uniformly. In order to suppressthe occurrence of unevenness in the concentration of the Compound A inthe cellulose fibers due to drying, the cellulose fibers in the shape ofa very thin sheet may be used, or a method of heat-drying orvacuum-drying the cellulose fibers, while kneading or stirring with theCompound A using a kneader or the like, may be employed.

As a heating device used for heat treatment, a device capable of alwaysdischarging moisture retained by slurry or moisture generated by anaddition reaction of phosphoric acid groups with hydroxy groups of thefiber to the outside of the device system is preferable, and forexample, forced convection ovens or the like are preferable. By alwaysdischarging moisture in the device system, in addition to being able tosuppress a hydrolysis reaction of phosphoric acid ester bonds, which isa reverse reaction of the phosphoric acid esterification, acidhydrolysis of sugar chains in the cellulose fibers may be suppressed aswell.

The time required for the heat treatment (heating time) is, althoughaffected by the heating temperature, preferably 1 second or more and 300minutes or less, more preferably 5 seconds or more and 270 minutes orless, and further preferably 10 seconds or more and 15000 seconds orless, after the phosphorylating agent has been mixed with cellulosefibers and the obtained mixture has been then exposed to the heatsource. For example, when the heating temperature is 100° C. or higherand 250° C. or lower, the heating time is preferably 10 seconds or more,more preferably 20 seconds or more, and further preferably 30 seconds ormore. When the heat treatment temperature is 100° C. or higher and 250°C. or lower, the upper limit of the heating time is preferably set at15000 seconds or less. In the present invention, by setting the heatingtreatment temperature and the heating time within an appropriate range,the amount of phosphoric acid groups introduced can be set within apreferred range.

The phosphoric acid group introduction step may be performed at leastonce, but may be repeated multiple times as well. This case ispreferable, since more phosphoric acid groups are introduced.

<Disintegration and/or Washing Steps>

After completion of the phosphoric acid group introduction step, it ispreferable to establish a disintegration step, and after completion ofthe disintegration step, it is preferable to further establish a washingstep. The disintegration step is carried out in accordance with JIS P8220. That is, the disintegration step is a step of converting thephosphorylated cellulose fibers obtained in the phosphoric acid groupintroduction step to a homogeneous pulp suspension. In this step, thephosphorylated cellulose fibers preferably have a size equivalent tocommon paper pulp (e.g., a width of 20 μm or more and 30 μm or less, anda length-average fiber length of 0.1 mm or more and 3.0 mm or less).When a suspension, in which the phosphorylated cellulose fibers arefully dispersed, can be obtained without performing such adisintegration step, the disintegration step may be omitted.

In the washing step, redundant chemicals such as a phosphorylating agentare washed away. In the washing step, it is preferable to repeat theoperation of subjecting the phosphorylated cellulose fibers aftercompletion of the disintegration treatment to filtration anddehydration, then pouring ion exchange water thereon, and then uniformlydispersing the solution by stirring.

<Alkali Treatment Step>

After completion of the phosphoric acid group introduction step and thedisintegration step, it is preferable to establish an alkali treatmentstep. By establishing such an alkali treatment step, the counterions ofthe phosphorylated cellulose fibers can be changed to various ions. Forexample, when sodium hydroxide is selected as alkali, the counterions ofthe phosphorylated cellulose fibers can be sodium ions. In the methodfor producing the cellulose fiber-containing composition of the presentinvention, the alkali treatment step may not be established, and in thiscase, one counterion of the phosphoric acid groups of the phosphorylatedcellulose fibers is an ammonium ion, and the other counterion thereof isa hydrogen ion.

The method of alkali treatment is not particularly limited, but a methodof immersing the phosphorylated cellulose fibers in an alkaline solutionis applied, for example.

The alkali compound contained in the alkaline solution is notparticularly limited, but it may be either an inorganic alkalinecompound or an organic alkali compound. The solvent of the alkalinesolution may be either water or an organic solvent. The solvent ispreferably a polar solvent (water, or a polar organic solvent such asalcohol), and the solvent may also be an aqueous solvent. Among alkalinesolutions, a sodium hydroxide aqueous solution, or a potassium hydroxideaqueous solution is particularly preferable, because of highversatility.

The temperature of the alkaline solution in the alkali treatment step isnot particularly limited, but it is preferably 5° C. or higher and 80°C. or lower, and more preferably 10° C. or higher and 60° C. or lower.

The immersion time in the alkaline solution in the alkali treatment stepis not particularly limited, but it is preferably 5 minutes or more and30 minutes or less, and more preferably 10 minutes or more and 20minutes or less.

The amount of the alkaline solution used in the alkali treatment is notparticularly limited, but it is preferably 100% by mass or more and100000% by mass or less, and more preferably 1000% by mass and 10000% bymass or less, with respect to the absolute dry mass of thephosphorylated cellulose fibers.

In order to reduce the amount of an alkaline solution used in the alkalitreatment step, the phosphorylated cellulose fibers may be washed withwater or an organic solvent before the alkali treatment step. After thealkali treatment, the alkali-treated phosphorylated cellulose fibers arepreferably washed with water or an organic solvent in order to improvethe handling property.

<Other Counterion Changing Treatments>

Counterions can also be changed by allowing the phosphorylated cellulosefibers to come into contact with inorganic alkali salts or organicalkali salts, instead of performing the aforementioned alkali treatmentstep. For example, when sodium chloride is selected as an inorganicalkali salt, the counterion of the phosphorylated cellulose fibers canbe set to be sodium. Alternatively, when alkyl ammonium chloride isselected as an organic alkali salt, the counterion of the phosphorylatedcellulose fibers can be set to be alkyl ammonium.

<Defibration Treatment>

When the cellulose fibers used in the present invention are ultrafinecellulose fibers having a fiber width of 1000 nm or less, a defibrationtreatment step may be established after completion of the alkalitreatment step In the defibration treatment step, fibers are defibratedusually using a defibration treatment apparatus to yield a slurrycomprising ultrafine cellulose fibers, and there is no particularrestriction on a treatment apparatus, or a treatment method.

A high-speed defibrator, a grinder (stone mill-type crusher), ahigh-pressure homogenizer, an ultrahigh-pressure homogenizer, ahigh-pressure collision-type crusher, a ball mill, a bead mill, or thelike can be used as the defibration treatment apparatus. Alternatively,for example, a wet milling apparatus such as a disc-type refiner, aconical refiner, a twin-screw kneader, an oscillation mill, a homomixerunder high-speed rotation, an ultrasonic disperser, or a beater may alsobe used as the defibration treatment apparatus. The defibrationtreatment apparatus is not limited to the above. Examples of a preferreddefibration treatment method include a high-speed defibrator, ahigh-pressure homogenizer, and an ultrahigh-pressure homogenizer, whichare less affected by milling media, and are free from apprehension ofcontamination.

<Sheet Formation Step>

Preferably, the method for producing the cellulose fiber-containingcomposition of the present invention further comprises a step of forminga sheet using the aforementioned phosphorylated cellulose fibers. Inthis case, the cellulose fiber-containing composition is preferably asheet-like non-woven fabric. In the step of forming a sheet usingphosphorylated cellulose fibers, the formation method can be selected,as appropriate, depending on the properties, shape, and the like of thesheet. In the present embodiment, methods such as, for example, a wetpaper-making method or a dry paper-making method can be adopted.

The step of forming a sheet using cellulose fibers may be a step offorming a sheet according to a wet paper-making method. Hereafter, anexample of the case of forming a sheet according to the wet paper-makingmethod will be described.

In the wet paper-making step, first, ion exchange water is added to thephosphorylated cellulose fibers obtained in the aforementioned step toobtain a cellulose fiber-containing slurry.

Then, the cellulose fiber-containing slurry is subjected to the wetpaper-making step. Examples of the paper machine used in the wetpaper-making step include a Fourdrinier paper machine, a twin-wire papermachine, a cylinder paper machine, an inclined wire paper machine, asingle net paper machine, and a Yankee paper machine. Moreover,paper-making may also be carried out using a handmade papermakingdevice.

The sheet obtained in the wet paper-making step is preferably subjectedto a dehydration drying step. In the dehydration step, dehydration maybe carried out by applying a pressure to the sheet. The pressure appliedherein is preferably 1 MPa or more, more preferably 5 MPa or more, andfurther preferably 10 MPa or more. On the other hand, the pressure ispreferably 100 MPa or less. Since the cellulose fiber-containingcomposition of the present invention is excellent in terms of resistanceto compression, a high-bulk sheet is easily obtained, even in a casewhere the pressure is applied under the above-described conditions inthe dehydration step.

In the case of using a dry paper-making method in the sheet formationstep, the phosphorylated cellulose fibers are uniformly mixed in theair, and the air current containing the phosphorylated cellulose fibersis then ejected onto a mesh-like endless belt comprising a suction boxin the lower side thereof, so as to form a sheet. Specifically, the drypaper-making method is a method comprising a step of mixing thephosphorylated cellulose fibers in the air and accumulating them. In thedry paper-making method, the above-described operations may be repeatedmultiple times, as necessary.

The method of drying the sheet in the dehydration drying step is notparticularly limited, and hot air, steam, infrared ray, microwave, etc.may be utilized, as appropriate. In addition, a method of directlycontacting a sheet with a metal plate or a metal roll used as a heattransfer medium, etc., may also be applied, as appropriate.

A pressurization operation may be further carried out on the sheet aftercompletion of the dehydration drying step. The pressure applied hereinis preferably 1 MPa or more, more preferably 5 MPa or more, and furtherpreferably 10 MPa or more. On the other hand, the pressure is preferably100 MPa or less. By this operation, the thickness and density of thesheet can be adjusted, as appropriate. Moreover, since the cellulosefiber-containing composition of the present invention is excellent interms of resistance to compression, a high-bulk sheet is easilyobtained, even in a case pressurization is carried out under theabove-described conditions.

(Intended Use)

The intended use of the cellulose fiber-containing composition of thepresent invention is not particularly limited. The cellulosefiber-containing composition is preferably a sheet, and more preferablya non-woven fabric. The cellulose fiber-containing composition isutilized, for example, in the state of a fluff pulp or a non-wovenfabric, as a component of absorbent articles for absorbing sweat, urine,menstrual blood, hazardous chemicals, etc., or is also utilized forsanitary papers, filter materials, buffer materials, etc.

EXAMPLES

The characteristics of the present invention will be more specificallydescribed in the following examples and comparative examples. Thematerials, used amounts, ratios, treatment contents, treatmentprocedures, etc. described in the following examples can beappropriately modified, unless they are deviated from the gist of thepresent invention. Accordingly, the scope of the present inventionshould not be restrictively interpreted by the following specificexamples.

Example 1 <Phosphorylation Reaction Step>

Pulp manufactured by Oji Paper Co., Ltd. (solid content: 96% by mass,basis weight: 213 g/m², sheet-shaped), which was needle bleached kraftpulp, was used as a raw material. 100 Parts by mass (absolute dry mass)of the needle bleached kraft pulp were impregnated with a mixed aqueoussolution of ammonium dihydrogen phosphate and urea, and were thencompressed to result in 45 parts by mass of the ammonium dihydrogenphosphate, 120 parts by mass of the urea, and 150 parts by mass of ionexchange water, so as to obtain a chemical-impregnated pulp. Theobtained chemical-impregnated pulp was subjected to drying and heatingtreatments in a hot-air dryer of 165° C. for 350 seconds, so thatphosphoric acid groups and phosphoric acid-crosslinked structures wereintroduced into cellulose in the pulp, thereby obtaining phosphorylatedcellulose fibers A.

<Disintegration and Washing Step>

ion exchange water was poured onto the obtained phosphorylated cellulosefibers A, so that the concentration of cellulose fibers became 2% bymass, and the resultant was then subjected to a disintegration treatmentfor 20 minutes, using a desk-top disintegrator with a volume of 2 L. Theobtained pulp slurry was subjected to filtration and dehydration toobtain a dehydrated sheet, and ion exchange water was poured onto thesheet again, followed by stirring for uniform dispersion. By repeatingthis operation, redundant chemicals were fully washed away, so as toobtain phosphorylated cellulose fibers B.

<Alkali Treatment Step>

The obtained phosphorylated cellulose fibers B were diluted with ionexchange water, so that the concentration of cellulose fibers became 2%by mass, and thereafter, a 1 N sodium hydroxide aqueous solution wasgradually added to the resulting phosphorylated cellulose fibers B,while stirring, so as to obtain a pulp slurry having a pH value of12±0.2. Thereafter, this pulp slurry was dehydrated to obtain adehydrated sheet, and ion exchange water was then poured onto the sheetagain, followed by stirring for uniform dispersion. The resultant wassubjected to filtration and dehydration to obtain a dehydrated sheet. Byrepeating this operation, redundant sodium hydroxide was fully washedaway, so as to obtain phosphorylated cellulose fibers C comprisingphosphorylated cellulose. Subsequently, according to the after-mentionedmethod, the water retention capacity of the phosphorylated cellulosefibers C was measured. Moreover, according to the after-mentionedmethod, the amount of phosphoric acid groups introduced into thephosphorylated cellulose fibers C and the content of crosslinkedstructures were measured.

<Sheet Formation and Pressing Step>

Ion exchange water was poured onto the obtained cellulose fibers C, sothat the concentration of cellulose fibers became 0.3% by mass, andthereafter, the resultant was subjected to dehydration and filtration toobtain a cellulose fiber-containing sheet having an area of 0.0043 m²and a basis weight of 200 g/m². This cellulose fiber-containing sheetwas dried in a humidity conditioning chamber having a temperature of 23°C. and a relative humidity of 50%, until the weight became constant.Subsequently, the resulting sheet was pressed at a pressure of 11.57 MPafor 60 seconds to obtain a cellulose fiber-containing sheet A (cellulosefiber-containing composition). By measuring the thickness of the pressedsheet, the density of the cellulose fiber-containing sheet A wascalculated. The density of the sheet was calculated in accordance withJIS P 8118: 1998. As a paper thickness gauge, a high-bridge paperthickness gauge (No. 735; manufactured by TAKAHASHI SEISAKUSHO Ltd.) wasused. In addition, according to the after-mentioned method, the watercontent (moisture content) and water-absorbing rate of the cellulosefiber-containing sheet A were measured.

Example 2

Phosphorylated cellulose fibers and a cellulose fiber-containing sheetwere obtained in the same manner as that of Example 1, with theexception that the aforementioned <Alkali treatment step> was notcarried out. The obtained phosphorylated cellulose fibers and cellulosefiber-containing sheet were subjected to the same measurements as thosecarried out in Example 1.

Example 3

Phosphorylated cellulose fibers and a cellulose fiber-containing sheetwere obtained in the same manner as that of Example 1, with theexceptions that the time required for the drying and heat treatment wasset at 300 seconds in the aforementioned <Phosphorylation reactionstep>, and further that the time required for the disintegrationtreatment using a disintegrator was set at 15 minutes in theaforementioned <Disintegration and washing step>. The obtainedphosphorylated cellulose fibers and cellulose fiber-containing sheetwere subjected to the same measurements as those carried out in Example1.

Example 4

Phosphorylated cellulose fibers and a cellulose fiber-containing sheetwere obtained in the same manner as that of Example 3, with theexception that the aforementioned <Alkali treatment step> was notcarried out. The obtained phosphorylated cellulose fibers and cellulosefiber-containing sheet were subjected to the same measurements as thosecarried out in Example 1.

Comparative Example 1 <Disintegration Step>

Pulp manufactured by Oji Paper Co., Ltd. (solid content: 96% by mass,basis weight: 213 g/m², sheet-shaped), which was needle bleached kraftpulp, was used as a raw material. Ion exchange water was poured thereon,so that the concentration of cellulose fibers became 2% by mass, and theresultant was then subjected to a disintegration treatment for 5minutes, using a desk-top disintegrator with a volume of 2 L. Theobtained pulp slurry was subjected to filtration and dehydration toobtain cellulose fibers A′. According to the after-mentioned method, thewater retention capacity of the cellulose fibers A′ was measured.

<Sheet Formation and Pressing Step>

Ion exchange water was poured onto the obtained cellulose fibers A′, sothat the concentration of cellulose fibers became 0.3% by mass, andthereafter, the resultant was subjected to dehydration and filtration toobtain a cellulose fiber-containing sheet having an area of 0.0043 m²and a basis weight of 200 g/m². This cellulose fiber-containing sheetwas dried in a humidity conditioning chamber having a temperature of 23°C. and a relative humidity of 50%, until the weight became constant.Subsequently, the resulting sheet was pressed at a pressure of 11.57 MPafor 60 seconds to obtain cellulose fiber-containing sheet A′. Bymeasuring the thickness of the pressed sheet, the density of thecellulose fiber-containing sheet A′ was calculated. Moreover, accordingto the after-mentioned methods, the water content (moisture content) andwater-absorbing rate of the cellulose fiber-containing sheet A′ weremeasured.

Comparative Example 2

Phosphorylated cellulose fibers and a cellulose fiber-containing sheetwere obtained in the same manner as that of Example 1, with theexceptions that the time required for the drying and heat treatment wasset at 200 seconds in the aforementioned <Phosphorylation reactionstep>, and further that the treatment using a disintegrator was notcarried out in the aforementioned <Disintegration and washing step>. Theobtained phosphorylated cellulose fibers and cellulose fiber-containingsheet were subjected to the same measurements as those carried out inExample 1. It is to be noted that, in Comparative Example 2, afteraddition of ion exchange water, the phosphorylated cellulose fibers wereuniformly dispersed in water only by gently stirring the solution byhand, and thus, a disintegrator was not used.

Comparative Example 3

Phosphorylated cellulose fibers and a cellulose fiber-containing sheetwere obtained in the same manner as that of Comparative Example 2, withthe exception that the aforementioned <Alkali treatment step> was notcarried out. The obtained phosphorylated cellulose fibers and cellulosefiber-containing sheet were subjected to the same measurements as thosecarried out in Example 1. It is to be noted that, in Comparative Example3, after addition of ion exchange water, the phosphorylated cellulosefibers were uniformly dispersed in water only by gently stirring thesolution by hand, and thus, a disintegrator was not used.

(Analysis and Evaluation) <Measurement of Water Retention Capacity>

The water retention capacity of cellulose fibers was measured inaccordance with SCAN-C 62:00. Upon the measurement of the waterretention capacity of cellulose fibers, the cellulose fibers weresubjected to a centrifugation treatment for 15 minutes under conditionsof 20° C. and 4400 rpm (weight acceleration upon centrifugation: 3950g). The amount of cellulose fibers subjected to the centrifugationtreatment was 0.5 g (added basis weight: 1700±100 g/m²) for a singlemeasurement. As a centrifugal separator, H-3R manufactured by KOKUSANCo. Ltd. was used. The water retention capacity was calculated accordingto the following equation:

Water retention capacity (%)=(weight of cellulose fibers aftercentrifugation treatment−absolute dry weight of cellulosefibers)/absolute dry weight of cellulose fibers×100.

It is to be noted that the larger the numerical value of water retentioncapacity, the higher the affinity of cellulose fibers with water thatcan be obtained.

<Measurement of Amount of Phosphoric Acid Groups Introduced>

The amount of phosphoric acid groups introduced was measured by aneutralization titration method. Specifically, the phosphoric acidgroups contained in cellulose fibers were completely converted to acidtype groups, and fibrillation was then performed by a mechanicaltreatment step (fibrillation step). Thereafter, while a 0.1 N sodiumhydroxide aqueous solution was added to the obtained ultrafine cellulosefiber-containing slurry, changes in the pH of the slurry (dispersion)were obtained, so that the amount of the phosphoric acid groupsintroduced was measured.

Conversion of the phosphoric acid groups to acid type groups was carriedout by diluting the obtained phosphorylated cellulose fibers with ionexchange water, so that the concentration of cellulose fibers became 2%by mass, and then gradually adding a sufficient amount of 1 Nhydrochloric acid aqueous solution to the resulting phosphorylatedcellulose fibers, while stirring. Subsequently, this cellulosefiber-containing slurry was stirred for 15 minutes and was thendehydrated to obtain a dehydrated sheet. Thereafter, by repeating theoperation of diluting the sheet with ion exchange water again, thenadding a 1 N hydrochloric acid aqueous solution thereto, the phosphoricacid groups contained in the cellulose fibers were completely convertedto acid type groups. Further, by repeating the operation of stirringthis cellulose fiber-containing slurry for uniform dispersion, and thensubjecting to filtration and dehydration to obtain a dehydrated sheet,redundant hydrochloric acid was fully washed away.

In the mechanical treatment step, ion exchange water was poured onto theobtained dehydrated sheet to obtain a cellulose fiber-containing slurry,in which the concentration of cellulose fibers was 0.3% by mass, andthis slurry was then treated using a defibration treatment device(manufactured by M Technique Co., Ltd., CLEARMIX-2.2S) under conditionsof 21500 rotations/min for 30 minutes. In the titration using alkali,changes in the pH values indicated by the dispersion were measured whileadding a 0.1 N sodium hydroxide aqueous solution to the ultrafinecellulose fiber-containing slurry.

In this neutralization titration, in a curve obtained by plotting pHvalues measured with respect to the amount of alkali added, two points,in which the increment (the derivative of pH to the amount of alkaliadded dropwise) becomes maximum, are given (i.e., a point in which theincrement becomes maximum, and a point in which the increment becomessecond maximum). Among these, the amount of alkali required until themaximum point of the increment obtained first after addition of alkali(hereinafter referred to as a “first end point”) is equal to the amountof strongly acidic groups in the dispersion used in the titration, andthe amount of alkali required until the maximum point of the incrementobtained second after addition of alkali (hereinafter referred to as a“second end point”) is equal to the amount of weakly acidic groups inthe dispersion used in the titration.

The alkali amount (mmol) required until the first end point was dividedby the solid content (g) in the dispersion to be titrated, to obtain afirst dissociated alkali amount (mmol/g). This amount was defined to bethe amount of phosphoric acid groups introduced.

<Measurement of Number of Crosslinking Points>

It is considered that a crosslinked structure is formed by dehydrationcondensation of phosphoric acid groups introduced into cellulose fibers.Specifically, the crosslinked structure is a structure in which glucoseunits of cellulose individually bind to each one of two P atoms ofpyrophosphoric acid via an O atom. Accordingly, when such a crosslinkedstructure is formed, weakly acidic groups are apparently lost, and thus,the amount of alkali required up to the second end point is reduced incomparison to the amount of alkali required up to the first end point.Specifically, the number of crosslinking points is equal to a valueobtained by dividing a different between the amount of alkali requiredup to the first end point (first dissociated alkali amount) and theamount of alkali required up to the second end point (second dissociatedalkali amount) by 2.

<Measurement of Water Content>

With regard to the water content, the weight of a cellulosefiber-containing sheet, which had been dried up to an equilibrium statein a humidity conditioning chamber at 23° C. and a relative humidity of50%, was measured. Thereafter, the cellulose fiber-containing sheet wasdried at 105° C. overnight, and the weight of the resulting cellulosefiber-containing sheet was then measured. After that, the water contentwas calculated according to the following equation:

Water content (%)=(weight of cellulose fiber-containing sheet beforedrying at 105° C.−weight of cellulose fiber-containing sheet afterdrying at 105° C.)/weight of cellulose fiber-containing sheet beforedrying at 105° C.×100.

<Measurement of Water-Absorbing Rate>

With regard to the water-absorbing rate, the cellulose fiber-containingsheet was cut into a rectangular sample having a width of 5 mm and alength of 50 mm, and an edge region ranging from the end of thisrectangular sample in the longitudinal direction to 5 mm from the endwas then immersed in ion exchange water (electrical conductivity: 2pS/cm or less). Thereafter, the time required for the ion exchange waterto reach from the end of the longitudinal direction to a distance of 45mm in the longitudinal direction was measured. After that, awater-absorbing rate (mm/sec) was calculated according to the followingEquation (2):

Water-absorbing rate (mm/sec)=40 (mm)/t (sec)  Equation (2).

In the above Equation (2), t represents the time (see) required for theion exchange water to reach from the end of the rectangular sample inthe longitudinal direction to a distance of 45 mm in the longitudinaldirection.

TABLE 1 Production conditions Physical properties of pulp Physicalproperties of composition Heating First dissociated Second dissociated(non-woven fabric) time [s] alkali amount alkali amount Number of WaterWater- Phosphor- upon (amount of strongly (amount of weakly crosslinkingreten- Water Den- absorbing ylation pro- Coun- acidic groups) P1 acidicgroups) P2 points (P1 − P2)/2 tion content sity rate step duction terion[mmol/g] [mmol/g] [mmol/g] capacity [mass %] [g/cm³] [mm/s] Ex. 1 Yes350 Na 2.25 1.65 0.30 260 13 0.66 5.41 Ex. 2 NH₄ 175 7 0.59 5.10 Ex. 3Yes 300 Na 2.01 1.51 0.25 334 13 0.69 3.55 Ex. 4 NH₄ 231 8 0.66 3.77Comp. No Not Not 0 0 0 103 5 0.71 2.09 Ex.1 heated heated (Not measured)(Not measured) (Not measured) Comp. Yes 200 Na 1.31 0.93 0.19 537 110.64 1.68 Ex. 2 Comp. NH₄ 280 8 0.66 1.68 Ex. 3

The cellulose fibers obtained in the Examples had high water retentioncapacity, and the cellulose fiber-containing sheets exhibited anexcellent water-absorbing rate. Although the cellulose fiber-containingsheets obtained in the Examples were high-bulk sheets (low-densitysheets), these sheets achieved both excellent water-retaining abilityand a high water-absorbing rate.

1. A composition comprising cellulose fibers having phosphoric acidgroups or phosphoric acid group-derived substituents, wherein in atleast a part of the cellulose fibers, the phosphoric acid groups or thephosphoric acid group-derived substituents are crosslinked, the numberof crosslinking points in the cellulose fibers, which is calculatedaccording to the following Equation (1), is 0.20 mmol/g or more, and thewater content is 50% by mass or less, with respect to the total mass ofthe composition:Number of crosslinking points=(amount of strongly acidic groupscontained in cellulose fibers−amount of weakly acidic groups containedin cellulose fibers)/2  Equation (1).
 2. The composition according toclaim 1, which is a non-woven fabric.
 3. The composition according toclaim 1, wherein when the composition is processed into a rectangularsample having a width of 5 mm and a length of 50 mm, then, an edgeregion ranging from the end of the rectangular sample in thelongitudinal direction to 5 mm from the end is immersed in ion exchangewater (electrical conductivity: 2 μS/cm or less), and then, the timerequired for the ion exchange water to reach from the end of thelongitudinal direction to a distance of 45 mm in the longitudinaldirection is measured, a water-absorbing rate (mm/sec), which iscalculated according to the following Equation (2), is 2.5 mm/sec ormore and 100 mm/sec or less:Water-absorbing rate (mm/sec)=40 (mm)/t (sec)  Equation (2) wherein trepresents the time (sec) required for the ion exchange water to reachfrom the end of the rectangular sample in the longitudinal direction toa distance of 45 mm in the longitudinal direction.
 4. The compositionaccording to claim 1, wherein the amount of the strongly acidic groupscontained in the cellulose fibers is 1.60 mmol/g or more.
 5. Thecomposition according to claim 1, wherein the water retention capacity(%) of the cellulose fibers, which is calculated according to thefollowing equation, is 150% or more:Water retention capacity (%)=(weight of cellulose fibers aftercentrifugation treatment−absolute dry weight of cellulosefibers)/absolute dry weight of cellulose fibers×100, wherein, in theabove equation, the water retention capacity is measured in accordancewith SCAN-C 62:00, and conditions for the centrifugation treatment aredetermined to be 20° C. and weight acceleration upon the centrifugationof 3950 g, and 15 minutes.